Sweeney Todd, Trafalgar Studios, London<br></br>Camelot, Open Air Theatre, London<br></br>Marieluise, Gate, London

Remember that old classroom exercise of laboriously counting up the images of night in Macbeth? Cripes, the characters mutter, gloomy, innit? Bit dark, wouldn't you say? Brrr, I'm frozen. And then you realise that as plays would have been put on in the afternoon, this was just Shakespeare's way of getting you in the mood. It's night, get it? And so it is for Sweeney Todd, deep in the bowels of the Whitehall Theatre. The set is black. The costumes are black. There's a large coffin centre stage. A cast mustered largely from the goth fraternity starts singing about how jolly frightening it all is. We're here to talk about death and night and gloom and cold, and to banish with collective will-power the knowledge that we're stuck in a typically underventilated London theatre on a hot summer evening.

I suppose it's testament to Stephen Sondheim's daring that the piece retains its ability to shock. Or rather - it still seems a weird topic to write a musical about. In 100 years will people be chortling over the antics of that naughty Dr Shipman? I had always assumed Sweeney was an urban myth, but the programme notes give a detailed account of his crimes and a firm execution date. (In a reversal of the Fred West case, it was Todd's female accomplice, Mrs Lovett, who topped herself before the trial.) The musical stresses the mitigating circumstances: Todd's wife and daughter have been corrupted and ruined by an evil judge. His planned revenge is thwarted by a blackmailing barbershop rival who needs to be eliminated, and it's Mrs Lovett, owner of the pie shop next door, who spots the obvious, thrifty solution. Before you know it, they're extolling the virtues of priest pie (as opposed to politician: so oily, you serve it with a doily).

There is no pit orchestra and the musical accompaniment is provided by the actors. Their sheer virtuosity and dexterity - all of them are constantly on the move, playing or singing throughout the show - is the most impressive aspect of the production. At one point, the mighty-lunged Karen Mann (as Mrs Lovett) sings and accompanies herself on the trumpet with the odd spare breath.

The ensemble playing is very strong, yet that's what the cast remains: an ensemble. True, Mann wrung the maximum comic effect from her ghoulish role, and Sam Kenyon's wraith-like Tobias radiated intensity, yet there was no real star on stage; no one I couldn't take my eyes off. Which brings me to Paul Hegarty's leather-jacketed Sweeney Todd. It's a strong-voiced, muscular performance, tilted well away from the grotesque, but what's missing is any sense of the audience's delighted complicity in his crimes. It's difficult to thrill to this Sweeney; he's not so much the embodiment of our darkest fears and desires as a rather cross and unlucky man.

By comparison, the Lerner and Loewe musical Camelot is a footling show, as fluffy as thistledown wafting across the open air stage at Regent's Park; yet the central trio were compulsively watchable. Lauren Ward's sweet and flighty Jenny and Matt Ward's pompous, then humbled, Lancelot complemented Daniel Flynn's masterly Arthur perfectly. The belly laughs might go to Russ Abbot's vague King Pellinore ("Questing beast, questing beast," he mutters), but the night belongs to Flynn: looking like a young Richard Harris (a previous incumbent in the role), he moves from callow youth to hardened warrior in two hours, and his confession that he loves both errant wife and friend too much to punish them is deeply moving. All the more so coming after such delicious silliness as the knights' indignant song, "Fie on goodness, fie!", as the implications of Lancelot's Christian chivalry sink in.

"Fie on men, fie!" would have leavened proceedings in the Gate's serious-minded Marieluise. Kerstin Specht's play about the life of the German playwright Marieluise Fleisser places her firmly in the Janet Frame/Sylvia Plath mould. Born in 1901 in Ingolstadt, the gifted imaginative girl could not fit in with family, town or school. As a young writer she was taken up, and as swiftly dropped, by Bertolt Brecht. Her work was scorned, and total mental collapse followed. It took a new generation of writers, among them Fassbinder, to appreciate her genius. She died in 1971.

The story is sombre, but Erica Whyman's direction, at least, is sparkling. Audience and actors are inside a sort of half-built IKEA wardrobe; there are doors which fly open, stairs to hurtle down, scaffolding to hang over. There are puppets, adults pretending to be children and a lot of rushing about. Technically it's very skilled, and the 22 numbered scenes fly past. Still, there's something old-hat about this play. While I'm sure that Nazi Germany wasn't a wholly supportive place for women writers, and it's no stretch to imagine that Brecht could be a bit of a pig, this woman-driven-to-madness-by-an-unfeeling-world theme has been done to death, as has the jack-in-the-box stagecraft.

Perhaps the play mirrors Fleisser's own work. If so, it's a strange mixture of the inspired and the clunky. There are chunks of historical exposition: "Versailles ruined us. We've been a garrison town since 1870. With no soldiers, there's no growth," remarks Marieluise's father. "There's been a coup. Hitler's taken control. They're arresting prominent Jews," a passerby helpfully sums up.

Yet Catherine Kanter is an impassioned (if slightly pop-eyed) Marieluise and Chris Myles a brusque, impressive Brecht. I especially admired Yana Yenizic, playing in turn the erotic dancer Anita Berber, a nun, and the River Danube (yes, really). Now that's what I call watchable.

'Sweeney Todd': Trafalgar Studios, London SW1 (0870 060 6632), to 9 October; 'Camelot': Open Air Theatre, Regent's Park, London NW1 (08700 601 811), to 4 September; 'Marieluise': Gate, London W11 (020 7229 0706), to 14 August